1. Field of the Invention
The present invention relates to a master information carrier used for recording information signals such as preformat information on a magnetic disk. In particular, the present invention relates to a master information carrier that includes a high-level region formed on a base, the high-level region being made higher than the other regions and including a ferromagnetic thin film pattern in correspondence with the information signals. Moreover, the present invention relates to a method for manufacturing a magnetic disk with the master information carrier.
2. Description of the Related Art
The recording density of a magnetic recording/reproducing device is increasing to achieve small size and large capacity. In the field of a hard disk drive, which is a typical magnetic recording/reproducing device, an areal recording density of more than 20 Gbits/in2 (31.0 Mbits/mm2) is already available on the market, and the technology proceeds with such a rapid pace that an areal recording density of 40 Gbits/in2 (62.0 Mbits/mm2) is expected within a couple of years.
Part of the technical background that has enabled such a high recording density is an increase in linear recording density resulting from the improvements in a magnetic recording medium and head-disk interface performance and new signal processing methods such as partial response.
However, the rate of increase in track density exceeds the rate of increase in linear recording density in recent years, and thus becomes a primary factor of an increase in areal recording density. This is because a magnetoresistive-type head, whose reproduction output performance is superior to that of a conventional inductive-type head, has been put to practical use.
At present, it is possible to reproduce signals from tracks having a width of not more than several micrometers with a high S/N ratio by using a magnetoresistive-type head. Moreover, it is expected that, with a further improvement in head performance, the track pitch will reach the submicron range in the near future.
To reproduce a signal with a high S/N ratio by scanning such a narrow track precisely with a magnetic head, the tracking servo technique for the magnetic head plays an important role. A current hard disk drive has preformat recording regions that are disposed at predetermined angular intervals over the entire circumference of a magnetic disk (magnetic recording medium), i.e., 360 degrees, and signals such as a tracking servo signal, an address information signal and a reproduction clock signal are recorded in the preformat recording regions. A magnetic head can monitor its position by reproducing these signals at predetermined intervals and scan a track precisely while correcting any displacement in the radial direction of the magnetic disk.
The preformat information signals, such as the tracking servo signal, the address information signal, and the reproduction clock signal, serve as reference signals for precisely scanning a track with the magnetic head. Therefore, the track positioning accuracy is required for recording these signals.
The following is a brief explanation of the technical and historical development of a master information carrier that is addressed by the present invention.
(1) First Stage: Use of Special Servo-track Recording Device
A current hard disk drive records the tracking servo signal, the address information signal, and the reproduction clock signal etc. with a unique magnetic head built into the drive by using a special servo-track recording device after a magnetic recording medium (hard disk) and the magnetic head have been incorporated into the drive. In this case, the necessary track positioning accuracy can be achieved by performing preformat recording while precisely controlling the position of the unique magnetic head built into the drive with an external actuator provided in the servo-track recording device.
However, the conventional technique for performing preformat recording with the unique magnetic head built into the drive by using the special serve-track recording device has the following problems.
First, the above recording method is “linear recording with a relative movement.” That is, recording with a magnetic head basically is linear recording achieved by the relative movement between a magnetic head and a magnetic recording medium. Therefore, the method in which the position of the magnetic head is controlled precisely by using the special servo-track recording device requires a lot of time for preformat recording.
Moreover, the use of the special servo-track recording device also is a problem. Since the special serve-track recording device is quite expensive, the cost needed for preformat recording is increased.
This problem becomes more conspicuous as the track density of a magnetic recording/reproducing device is improved. In addition to the increased number of tracks in the radial direction of a disk, the track density also is improved with an increase in recording density. Thus, it is necessary to achieve higher positioning accuracy of a magnetic head. Therefore, the angular intervals per one revolution of the disk should be reduced, where the preformat recording regions for recording information signals such as the tracking servo signal are provided. Accordingly, the amount of signals to be preformat-recorded on the magnetic disk increases with an increase in recording density of the device, so that a lot of time is required for preformat recording.
Although there is a tendency to reduce the diameter of a magnetic disk medium, e.g., to 2.5 inches or 1.8 inches, the demand for disks having a large diameter of 3.5 inches or 5 inches is still great. The amount of signals to be preformat-recorded increases with an increase in recording area of the disk. Therefore, the cost performance of such large-diameter disks is affected significantly by the time required for preformat recording.
Second, there is a problem about “dynamic linear recording with a relative movement.” That is, a recording magnetic field is broadened (a) by a spacing between a magnetic head and a magnetic recording medium and (b) by the pole shape at the tip of the magnetic head. Therefore, the magnetization transition lacks sharpness at the ends of a track on which signals are preformat-recorded.
Since recording with a magnetic head basically is dynamic linear recording achieved by the relative movement between a magnetic head and a magnetic recording medium, a certain amount of spacing has to be provided between the magnetic head and the magnetic recording medium in view of interface performance therebetween. Moreover, the pole shape at the tip of the current magnetic head includes two elements that are used separately for recording and reproduction. Therefore, a pole width on the trailing edge side of a recording gap corresponds to a recording track width, and a pole width on the leading edge side is at least twice as large as the recording track width. Such a large pole on the leading edge side also serves to shield a MR head for reproduction.
Both the spacing and the pole shape at the tip of the magnetic head cause the recording magnetic field to broaden at the ends of a recording track. As a result, the magnetization transition lacks in sharpness at the ends of a recording track on which signals are preformat-recorded, or erased regions are formed on both ends of the track.
In the current tracking servo technique, the position of a magnetic head is detected based on the amount of change in reproduction output when the magnetic head deviates from the track to be scanned. Therefore, as with the case where data signals recorded between the servo areas are reproduced, it is necessary to achieve not only an excellent SIN ratio when the magnetic head scans a track precisely but also a sharp change in reproduction output when the magnetic head deviates from the track to be scanned, i.e., a sharp off-track characteristic.
Thus, a lack of sharpness in magnetization transition at the ends of a track on which signals are preformat-recorded makes it difficult to achieve precise tracking servo technique that will be used in recording signals on submicron tracks in the future.
(2) Second Stage: Introduction of Master Information Carrier (Transfer Recording Technique)
To solve the two problems in preformat recording with a magnetic head, there is an idea of using a master information carrier as an original master. The master information carrier includes a base on which a ferromagnetic thin film pattern that corresponds to preformat information signals is formed.
JP 10(1998)-40544 A discloses the following technique: bringing the surface of a master information carrier into contact with the surface of a magnetic recording medium; magnetizing a ferromagnetic thin film pattern that is formed on the master information carrier so as to correspond to information signals; and transferring and recording a magnetization pattern that corresponds to the ferromagnetic thin film pattern onto the magnetic recording medium.
This preformat recording technique makes it possible to perform favorable and efficient preformat recording without sacrificing other important performances such as the S/N ratio and the interface performance of the recording medium.
To make the master information recording technique disclosed in JP 10(1998)-40544 A effective, it is necessary to ensure uniform contact between the ferromagnetic thin film pattern and the magnetic recording medium during transfer recording.
(3) Third Stage: Further Improvement in Master Information Carrier (High Level Region and Low Level Region)
JP 10(1998)-269566 A discloses the following technique: at least a portion of the surface of a region including no ferromagnetic thin film pattern is made lower than that of a region including the ferromagnetic thin film pattern that corresponds to information signals. The higher region where the ferromagnetic thin film pattern that corresponds to information signals is formed is referred to as a high-level region, while the lower region where no ferromagnetic thin film pattern is formed is referred to as a low-level region.
According to this technique, only the high-level region of a master information carrier can be brought into contact with a magnetic recording medium and the low-level region is not in contact with the magnetic recording medium. In other words, the contact with the magnetic recording medium is not made over the entire surface of the master information carrier, but in part thereof. Thus, improved contact can be established between the ferromagnetic thin film pattern that corresponds to information signals and the magnetic recording medium. Moreover, when the master information carrier is brought into contact with the magnetic recording medium, a gas contained in a space between the low-level region and the magnetic recording medium is exhausted so as to generate negative pressure. Thus, the close contact between the high-level region and the magnetic recording medium further can be enhanced by the action of atmospheric pressure.
FIGS. 9A and 9B show an example-of the above master information carrier. FIG. 9A is a plan view of the master information carrier, and FIG. 9B is an enlarged cross-sectional view taken along the alternate long and short dashed line 9B—9B of FIG. 9A. In FIG. 9A, a master information carrier 41 is substantially circular in shape and has an orientation flat 41a. The master information carrier 41 includes a high-level region 42 where a ferromagnetic thin film pattern 42p (indicated by hatching) is formed so as to correspond to preformat information signals, and a low-level region 43 where no ferromagnetic thin film pattern is formed. The surface level of the low-level region 43 is made lower than that of the high-level region 42, so that the master information carrier 41 has an uneven surface.
When the master information carrier 41 with such an uneven surface is used to record preformat information signals on a magnetic disk, the master information carrier 41 should come into close contact with a magnetic disk 51 (indicated by the alternate long and two short dashed line in FIG. 9B). At this time, an exhaust path 45 is formed according to a difference in height between the high-level region 42 (having a height of h1) and the low-level region 43 (having a height of h2). By exhausting a gas contained in this exhaust path 45 from a hole of the magnetic disk, negative pressure is generated in the low-level region 43, and thus the high-level region 42 uniformly contacts the magnetic disk 51 due to the action of atmospheric pressure. Then, an external magnetic field is applied while maintaining the close contact, so that the preformat information signals corresponding to the ferromagnetic thin film pattern 42p are transferred and recorded onto a magnetic recording layer formed on the surface of the magnetic disk 51.
When a gas contained in the exhaust path 45 between the low-level region 43 and the magnetic disk 51 is exhausted from the hole of the magnetic disk 51, larger negative pressure may be generated in the central portion of the master information carrier 41. Consequently, the portion of the master information carrier 41 that corresponds to the hole of the magnetic disk 51 may be drawn and deformed.
FIG. 10 is a cross-sectional view schematically showing an apparatus for performing the transfer recording of information signals onto a magnetic disk with a master information carrier. A magnetic disk 51, to which information signals are transferred, is held by a disk-supporting member 61. The disk-supporting member 61 includes a suction hole 61a, and an exhaust duct 61b is connected to the end portion of the suction hole 61a. Further, an exhaust device 61c is provided at the end portion of the exhaust duct 61b. This exhaust device 61c operates to produce negative pressure in a space between the magnetic disk 51 and the master information carrier 41 through the exhaust duct 61b, the suction hole 61a, and the hole 51b of the magnetic disk 51. Thus, the master information carrier 41 is drawn toward the magnetic disk 51. Although negative pressure also is generated in the low-level region located between the high-level regions of the master information carrier 41, larger negative pressure is exerted on the central portion of the master information carrier 41 that is opposite to the hole 51b of the magnetic disk 51. Therefore, the master information carrier 41 is deformed easily in the central portion by such high suction force. FIG. 10 is exaggerated for purposes of illustrating deformation of the master information carrier 41 with its central portion drawn. Since the base of the master information carrier 41 is a rigid body, the close contact between the high-level region 42 of the master information carrier 41 and the magnetic disk 51 may be degraded partly by the deformation.
In transfer recording that uses a master information carrier, it is very important to prevent deformation of the master information carrier when the high-level region including the ferromagnetic thin film pattern comes into close contact with a magnetic disk by exhausting a gas contained in a space between the low-level region and the magnetic disk.